This project seeks to determine the location and mechanism of neurotransmitter secretion. Rapid freezing and subsequent freeze-fracture of synapses capture fleeting structural changes in the cell membrane accompanying discharge of synaptic vesicles. By these means, the prodromata and aftermath of synaptic vesicle exocytosis have been determined. This approach has been extended to other secretory cells where details surrounding the initiation of secretion are more readily studied. New methods have been developed to use rapid freezing to determine how the distribution of intracellular calcium changes in different functional states. Organelles which store and release calcium during secretion as well as sequestering it afterwards have been found. This work is significant in that it defines the dynamic structure of normal synapses by relating normal variations in structure to different functional states. The current program also includes freeze-fracture of developing and degenerating synapses; the results will aid in understanding of normal development as well as the effects of diseases and developmental failures on synaptic structure in the brain and peripheral nervous system.